1. Field of the Invention
This invention relates to a process for the production of a cross-linked polyaspartic acid resin having (bio)degradability and high water absorbency and also to a process for the production of a cross-linked polysuccinimide useful as a precursor for the cross-linked polyaspartic acid resin.
2. Description of the Related Art
Technical Background of Superabsorbent Polymers
A superabsorbent polymer is a resin capable of absorbing water from several tens of times to several thousands of times its own weight, and is used in sanitary products, such as sanitary napkins and disposable diapers, and also in a variety of other fields.
Related Art on Superabsorbent Polymers
Known examples of superabsorbent polymers employed in such applications include partial neutralization products of cross-linked polyacrylic acids (JP Kokai No. 55-84304, U.S. Pat. No. 4,625,001), partial hydrolysis products of starch-acrylonitrile copolymers (JP Kokai 46-43995), starch-acrylic acid graft copolymers (JP Kokai 51-125468), hydrolyzation products of vinyl acetate-acrylate ester copolymers (JP Kokai 52-14689), cross-linked copolymers of 2-acrylamido-2-methylpropanesulfonic acid and acrylic acid (EP 0068189), cross-linked polymers of cationic monomers (U.S. Pat. No. 4,906,717), and hydrolysis products of cross-linked isobutylene-maleic anhydride copolymers (U.S. Pat. No. 4,389,513).
These superabsorbent polymers however are accompanied by the problem that they do not degrade following disposal after use.
Under the circumstances, these superabsorbent polymers are currently disposed of by incineration or reclamation. However, it is indicated that disposal in incinerators is a cause of global warming and acid rain in addition to a cause of damage to incinerator materials due to heat occurring during incineration. On the other hand, reclamation disposal is accompanied by problems such as poor stabilization of reclaimed grounds due to the bulky and undegradable nature of plastics and, moreover, is facing a serious problem in that there are no longer many sites suited for reclamation.
Described specifically, these polymers are poor in degradability and remain semipermanently in water or soil. Their disposal presents a very serious problem from the viewpoint of environmental preservation. For example, in the case of polymers for disposable applications, led by sanitary products such as disposable diapers and sanitary napkins, their recycling, if tried, would require substantial expenditure while their incineration, if attempted, would significantly affect the global environment due to the enormous quantities involved. On the other hand, it has been reported that use of a cross-linked polyacrylic acid resin as an agricultural and horticultural water-holding material leads to the formation of complexes with multivalent ions such as Ca.sup.2+ in soil and hence results in the formation of an insoluble layer [Matsumoto et al., KOBUNSHI (High Polymer, Japan), 42, August, 1993]. Such a layer is considered to have low toxicity by itself but is not found at all in the natural world. Nothing is known about their influence on the ecosystem resulting from an accumulation of such polymers in soil over a long period and therefore, a thorough investigation is needed. A cautious attitude towards their use is hence desirable. Likewise, non-ionic resins have a potential problem of accumulating in soil due to their undegradable nature although they do not form complexes. It is therefore likely that they would have adverse effects on the natural world.
Furthermore, these polymerized resins use monomers which are highly toxic to human skin and the like. A great deal of work has been conducted to eliminate such monomers from polymerized products. Nonetheless, their complete elimination is difficult. Still higher difficulties are expected especially in the production on an industrial scale.
Technical Background of Superabsorbent Polymers Having Biodegradability
On the other hand, biodegradable polymers have been attracting interest as "globe-compatible materials" in recent years. Their use as superabsorbent polymers has also been proposed.
Known examples of biodegradable superabsorbent polymers employed in such applications include cross-linked polyethylene oxide (JP Kokai 6-157795, etc.), cross-linked polyvinyl alcohol, cross-linked carboxymethylcellulose (U.S. Pat. No. 4,650,716), cross-linked alginic acid, cross-linked starches, and cross-linked polyamino acids. Among these, the cross-linked polyethylene oxide and cross-linked polyvinyl alcohol have small water absorption and are hence not particularly suited for use as materials in products requiring high water-absorbency such as sanitary products, disposable diapers, disposable dustcloths and paper towels.
Further, these compounds can be biodegraded only by certain particular bacteria, so that under general conditions, their biodegradation will be slow or will not take place at all. Moreover, the biodegradability will be reduced extremely as the molecular weight becomes greater.
In addition, cross-linked saccharides such as cross-linked carboxymethylcellulose, cross-linked alginic acid and cross-linked starches contain many firm hydrogen bonds in their molecules, thereby exhibiting strong interaction between molecules and/or polymers. Accordingly, molecular chains cannot be opened widely meaning that their water-absorbency is not high.
Technical Background of Polyamino Acid Superabsorbent Polymers
On the other hand, polymers which are available by cross-linking polyamino acids do have biodegradability and are thus compatible with the global environment. It has also been found that, even when absorbed in the body, they are digested and absorbed by enzymatic action and moreover, they do not exhibit antigenecity in the body and their metabolites are free of toxicity. These polymers are accordingly materials which are also safe for human beings.
As a disclosed example of such a polymer, a process for the production of a polymer having high water-absorbency, which comprises irradiating .gamma. rays to poly-.gamma.-glutamic acid, was reported by Kunioka et al. in KOBUNSHI RONBUNSHU (The Journal of the Society of Polymer Science, Japan), 50(10), 755 (1993). From an industrial viewpoint, however, a .sup.60 Co irradiation system for use in this technology requires considerable equipment for shielding radiation, and sufficient care is also required for its control. This technology is therefore not practical. As a further problem, the high cost of polyglutamic acid as the starting substance can also be mentioned.
In addition, processes for obtaining a hydrogel by cross-linking an acidic amino acid were reported by Akamatsu et al. in U.S. Pat. No. 3,948,863 (corres. JP Kokoku 52-41309) and Iwatsuki et al. in JP Kokai 5-279416. Further, use of cross-linked amino acid polymers as superabsorbent polymers was reported by Sikes et al. in JP PCT Kokai 6-506244 (corres. U.S. Pat. No. 5,247,068 and U.S. Pat. No. 5,284,936), Suzuki et al. in JP Kokai 7-309943 and Harada et al. in JP Kokai 8-59820.
In all the above reports, however, these polymers did not have sufficient water or saline absorbency and were not practically usable. Further, these resins are accompanied by a problem that their gels have low strength and become sticky with time.
Background of Technical Concept of the Present Inventors
As is described in JP Kokai 7-224163, the present inventors disclosed a technique for the production of a cross-linked polyaspartic acid resin having high saline absorbency, which comprises reacting a polysuccinimide with a cross-linking agent to hydrolyze remaining imide rings.
Further, as is disclosed in JP Kokai 9-169840, the present inventors also disclosed a technique for the production of a cross-linked polyaspartic acid resin having saline absorbency, which comprises crosslinking a polysuccinimide and then hydrolyzing remaining imide rings in an intimately mixed solvent of a water-miscible organic solvent and water.
Cross-linked polyaspartic acid resins available by these techniques are very useful for their globe-compatibility and high water absorbency. From the industrial viewpoint, however, there is room for further improvements. Described specifically, the concentrations of cross-linked polyaspartic acid resins produced in these production processes are as low as about 5 wt. %, indicating the existence of room for an improvement in volumetric efficiency. Further, according to the knowledge of the present inventors, hydrolysis of imide rings of a cross-linked polysuccinimide in water upon production of a cross-linked polyaspartic acid resin causes the resin in the reaction system to absorb water and swell, resulting in significant gelation. This makes it difficult to stir the reaction system. A hydrolyzing reagent can no longer spread so that the reaction is not allowed to proceed sufficiently. This results in a problem that the thus-produced cross-linked polyaspartic acid resin is not provided with high water absorbency. On the other hand, hydrolysis in an organic solvent or a mixed solvent of water and an organic solvent, said mixed solvent containing the organic solvent in a high proportion, tends to allow the resin in the reaction system to undergo a hydrolysis reaction only at surfaces of resin particles, so that the reaction velocity of the hydrolysis becomes very slow. This also results in the problem that the thus-produced cross-linked polyaspartic acid resin is not provided with high water absorbency.
The present inventors found that cross-linked polyaspartic acid resins produced as described above contain water-soluble impurities such as water-soluble polymers and salts. Accordingly, they have room for further improvements such as an improvement in gel strength, the elimination of stickiness on a gel surface and improvements in absorbency for salt-containing aqueous solutions.
In the production process of JP Kokai 7-224163, a failure in the full control of the production step of a cross-linked polysuccinimide may not allow the cross-linking to proceed sufficiently so that a superabsorbent resin having high water absorbency may not be obtained. According to the production process disclosed in JP Kokai 9-169840, the resin absorbs a reaction solvent during a cross-linking reaction in the production step of a cross-linked polysuccinimide and the reaction system is thus solidified. This makes it difficult to perform the reaction under stirring, or post-reaction treatments become extremely difficult. This production process is therefore not suited for industrial production. The production process of a cross-linked polysuccinimide as a precursor of a cross-linked polyaspartic acid resin has room for improvements in connection with simplification and improvements of process control and process steps themselves.
In JP Kokai 7-224163 and JP Kokai 9-169840, it is described that use of a basic amino acid, such as lysine or ornithine, as a polyamine for use as a cross-linking agent is preferred from the standpoint of the safety of the remaining unreacted cross-linking agent and decomposition products. However, the basic polyamino acids specifically disclosed in these publications involve problems such that they are low in reactivity and that, although use of their esters can provide improvements in reactivity, these esters themselves are costly.